The present invention relates to the art of metal surface treatment. More specifically, the present invention relates to the after-treatment of metals which have been subjected to a conversion coating step and to a novel polymeric after-treatment solution or dispersion for application to metals which have been treated with conversion coatings.
The need for applying protective coatings to metal surfaces for improved corrosion resistance and paint adhesion characteristics is well-known in the metal finishing and other metal arts. The usual technique for forming such protective coatings on metal surfaces involves contacting the metal surface with a solution containing phosphate ions to form a corrosion resistant, nonreactive, phosphate complex coating on the metal surface. Such coatings convert the metal surface from a chemically active surface readily susceptible to oxidation and corrosion and are known in the art as "conversion coatings".
The corrosion-resistance provided by conversion coatings can be enhanced by treating the phosphatized metal surface with an after-treatment solution such as a dilute aqueous acidic solution containing hexavalent chromium ions. However, although chromium after-treatment solutions and processes are known to be effective, there is a problem which arises from the effluents. Because of the toxic nature of hexavalent chromium compounds, expensive treatment equipment and processes must be used to remove the chromates from plant effluents in order to prevent the pollution of rivers, streams and drinking water sources. Hence, although the corrosion resistance and the paint adhesion characteristics of conversion coated metal surfaces can be enhanced by an after-treatment solution containing hexavalent chromium compounds, in recent years there have been research and development efforts directed to discovering effective alternative after-treatment systems to the known chromium containing after-treatment solutions.
One such class of compounds consists of derivatives of 4-vinylphenol in polymer form as disclosed in U.S. Pat. No. 4,433,015. However, the use of such monohydroxy phenyl compounds in the after-treatment of phosphatized metals presents a very real problem because of the difficulty in synthesizing such compounds in a form which will avoid the presence of significant amounts of unreacted phenol in the effluent. Such unreacted phenol presents contamination problems comparable to or even worse than the problems involved in the use of hexavalent chromium after-treatment compounds. The same is true of monohydroxy phenyl compounds of the type shown in U.S. Pat. Nos. 4,457,790 and 4,517,028. The problem in all such cases is that the process of making such monohydroxy compounds becomes very expensive if the pollution problems referred to above are to be avoided. Therefore, considerable effort has been devoted to developing after-treatment products which do not leave significant amounts of unreacted phenol in the effluent and at the same time provide after-treatment products which are easier and cheaper to manufacture than those heretofore known and which are easier to use.